What BN hardness lead are you using for your cast. If your using plain WW at BN12 or less then I wouldn't worry about the .379 dia slugs at all however if you are using commercial "hard cast " that is usually around BN 25-27 I would look for a different alloy...failing to find any I would probably talk myself into trying the hard cast as well at .379 LOL.
One thing you could try is something I read about before but never tried to verify until this winter. I have read that cast slugs will "cure-out" to a slightly smaller dia. over a period of time. I had a bunch of slugs cast over a year ago (.311, .379 and .459) that I powder coated then sized but never got around to using them, a couple of days ago I had a buddy that needed a few .311 slugs to try in his 30-30 so I dug last years castings out, I was surprised to find that every one of them miked out to .3095-.310. They had all "cured" to at least .001 under size in that year...I verified the same thing with all the rest of the slugs I have on hand of the previously mentioned dia.
so if you want to wait a year for your slugs to cure the .379 sized ones will probably end up at .378.